DIY Doctor UK DIY Forums

We're renovating our living-room and have stripped the walls down to the brick.

Before re-plastering I'd like to insulate the external walls (solid brick walls). I have spent the day trying to determine what is the best material to use and am none the wiser!

Ideally I'd like to know which material gives the best insulation, comparing like-for-like thickness.

I don't want to add too much thickness to the walls - perhaps 25mm max?

I rang the energy saving trust and they told me the best thing would be Sempatap thermal lining, but I'm not so sure. I don't have a way of comparing it with other materials. They told me the U value of it is 1.58 W/m2K.

The thermal conductivity (I don't know if that is the same thing as U value) of acoustic plasterboard is 0.24W/mK.

Celotex Celotex PIR Insulation Board (25mm) as sold at Wickes apparently has the same level of thermal insulation as almost twice as much glasswool insulation.

I saw that glasswool insulation has a thermal conductivity of 0.04 W/mK, so I'm guessing the Celotex has 0.02 W/mK? Which sounds much better than the Sempatap, assuming that U-value and thermal conductivity are the same thing.

I saw expanded polystyrene thermal board has thermal conductivity of 0.038 W/mK.

I saw somewhere that the thermal conductivity of ordinary plasterboard is 0.16 W/mK.

Now I'm writing this out it looks to me like the Celotex is the best option, assuming thermal conductivity and U-values are the same.

So here are my questions:

Are thermal conductivity and U-value the same thing?

If not, how do I convert from one to the other?

Does anyone have any tips for comparing different materials?

HilaryN,
To get to U value from thermal conductivity you need to divide by the thickness of the material.
TC of glass wool is 0.04 so 100 mm would give a U value of 0.04/0.1=0.4W/m2K. 250 mm would give 0.04/0.25=0.16.
I would not recommend you put anything other than a masonry type of coating with a breathable paint on an external solid brick wall. If you put any covering on the wall which traps moisture condensation this will form on the relatively cold wall. With a porous wall in cold conditions water will appear at the bottom edge of the wall, soaking carpets, rotting skirtings, wetting structural timbers fitted into the wall and associated problems.
end

“To get to U value from thermal conductivity you need to divide by the thickness of the material. “

Thanks, Stoneyboy, I'm very grateful for your explanation because now it's all fallen into place and makes perfect sense! It also explains why thermal conductivity is in W/mK and U value is in W/m²K. **

For my purposes, then, I should look at the thermal conductivity because I want to see which material is best for the same thickness.

So Sempatap (10mm thick) would have thermal conductivity of 1.58 * 0.01 = 0.0158 W/mK which makes it the best insulator out of all of them, with Celotex not far behind at 0.02 W/mK if my calcs are correct.

“I would not recommend you put anything other than a masonry type of coating with a breathable paint on an external solid brick wall. If you put any covering on the wall which traps moisture condensation this will form on the relatively cold wall. With a porous wall in cold conditions water will appear at the bottom edge of the wall, soaking carpets, rotting skirtings, wetting structural timbers fitted into the wall and associated problems. “

Yes, I saw you'd written that in another post and I feel a bit anxious as I certainly don't want to have problems in the future.

Are you saying that you don't recommend putting insulation on a solid brick wall?

But how will the insulation trap moisture?

The person at the company which sells Sempatap told me the wall should be skimmed first, so if there are no gaps between the wall and the insulation shouldn't that be OK? Similarly for thermal board?

Many TIA for answering my questions.

Hilary


** For others reading this, m2 should be meters squared, but I don't know how to format the 2 to look like a square as HTML doesn't seem to work here.
Ah, wait, Alt+0178 may do the trick!

HilaryN,
Are you saying that you don't recommend putting insulation on a solid brick wall? Yes, probably not what you wanted to hear.
You can skim and seal a solid wall which will stop moisture reaching the inner surface of the wall but only in the areas treated. Walls which join the outer wall and areas between floors will become wetter.
The main problem will be condensation, once a solid wall is covered with insulation it becomes colder and attracts more moisture. Even if a DPM is incorporated in the covering (eg plasterboard with integral foil dpm) damp in the vapour phase will still get to the cold wall through joins in the boards or around the edges of the boards. If the insulated outer wall abuts a carpeted concrete floor the condensation will show as a damp edge along the carpet.
end

"Yes, probably not what you wanted to hear. "
Right! :(

Thanks for that, though. I've written to the energy saving trust and National Insulation Association (with a link to this thread) to let them know, as they recommend insulating solid brick walls.

HilaryN,
Unfortunately there are many energy-saving measures being promoted without the long term consequences being considered.
Yes, you could insulated your solid wall and short term the wall and house will be warmer. I cannot say for definite that a damp problem will arise, there are too many variables eg water penetration of the external wall; temperature and humidity in your house; exposure and orientation of the external wall; outside temperature etc. In my experience insulating a solid exterior outside wall without measures to remove water/condensation can lead to serious damp problems.
Supposing you did carry out insulation and 5 years later you have serious rot problems with structural timber adjacent to the wall. Do you think either of the official bodies you have mentioned will put things right?

Another example of potential long term problems is the addition of extra insulation in a loft. If this done and cables are encased in the insulation this contravenes regulations and can be a safety issue.
The same issues can be applied to cavity wall insulation if this encases cables.
end

stoneyboy,

You're absolutely right about it being important to consider long-term consequences, and that's why I've written to the bodies concerned, because I think it's important that they are aware of the isssues you have mentioned before they make recommendations.

In my case I live in a terraced flat and the external wall in question isn't very big (and half of it is window anyway) so in the end it's not a big deal if I don't insulate it - I just thought it would be a good idea to save on heating bills. In light of your advice, I've reconsidered!

Hilary

If you decide to use a laminate the most thermally efficient product is phenolic foam. I know as I used to sell then several years ago.

I would recommend you speak to Encon Insulation. Ian (of Encon) is the most knowledgeable guy I know in the industry and he can do free calculations to determine if you have a potential condensation problem.

Two things I will say is phenolic is a great product and has great fire resistance properties however if you screw fix you will need stainless steel fixings as carbon steel will react to the phenolic or alternatively it would be cheaper to dot and dab it to the walls.

However please note phenolic may be the thinnest insulator but it is also one of the most expensive so you may wish to consider trading product thickness against cost, in which case Ian should be able to give you a few alternatives such as extruded polystyrene or polyurethane.

Regards

Jim Barrie
Government insulation grant advisor

There are many energy-saving measures being promoted while planning for any type of construction inside or outside.
I also agreed with stone boy the main problem will be condensation while construction. :P

Hi Hilary,

Which Insulation did you finally go?

Was it any good?

Regards,

Shahie

Hilary, I can understand your dilemma.

The key thing is water vapour!

Water vapour that is created by you, in your home by washing, cooking, breathing..........must be dealt with.

If you have an extractor fans in your kitchen and bathroom and if you use them, when and after cooking and washing than most of the water vapour will move outside, however a basic extractor fan is not recommended as they work by pushing the warm air from the home and by pulling cold possibly wet air from the outside through holes and cracks in the homes fabric.
Extractor fans with heat exchangers and built in humidistats are best as they swap equal amounts of wet inside air for outside air (but see below) with out dragging cold air into the home through the various gaps and cracks and causing drafts.

If you open the windows for ten minutes in the morning, then a lot of the water vapour will move outside and be replaced by colder drier air. This does not always happen as sometimes the air outside and the air inside are at, or are close to the same temperature and or humidity.

You can of course buy and use a large dehumidifier, this is guaranteed to work as it merely rotate the same air that is indoors removing the water vapour, keeping the same air moving but, does not provide fresh air or remove smells.

The final point is that water vapour like heat, always moves from hot to cold.

This means, that if you insulate your outside walls by sticking 25mm polystyrene to them, making sure that you cover the entire wall side of the polystyrene with plasterboard adhesive, taking care to leave no gaps, then you will improve your heat loss through the walls by about 90% and you will leave no gaps for water vapour to move through or condense in.

One of the two large manufacturers of insulation makes a proprietary insulation product that has plasterboard stuck to their version of polystyrene, if you use this it will then only require a skim coat of plaster to finish.

Water vapour is a very fine gas, that can move through most of the things that houses are made of.
But, because it mainly moves from hot to cold if it does enter a solid wall it will merely make its way through to the cold outside.

Hi,
I don't claim to be any sort of expert, I just found the suggestion that insulation leads to condensation interesting. Condensation forms on a colder surface as the moisture holding capacity of a cold surface (or cold air) is lower than a warm one, thus you get less condensation on the inside of a double glazed unit than a single glazed window. The problem is if the double glazing unit seal is broken then condensation will form on the inside of the unit between the pains. Likewise if a wall is better insulated it's internal surface temperature will be relatively warmer than if it was more poorly insulated, so logic would follow you would get less condensation forming on the inside surface. It would seem that the problem with most peoples attempts at insulating is that they leave air gaps/pockets between the insulation and the wall where condensation forms on the cold wall in the cavity and if the space is not ventilated the condensation will rot timber studwork, and rust steel screws & nails etc.
Alternatively come up with a system as with the cavity wall with a wet outer and dry inner wall that never touch. (I appreciate they still have metal wall ties joining them, but no system is perfect)

Thus to conclude if you can insulate a solid wall without leaving air gaps and try to avoid materials are adversely affected like timber & steel then condensation should be reduced, or am I wrong?

Andy :roll:

Lets go over this again.

Heat always moves to cold.

Water vapour always condenses onto a surface that is

colder than the air temperature that is holding the water

vapour.

Therefore, for every air temperature, that holds a certain

amount of water vapour, there is a lower surface or air

temperature that will chill the passing air, lower its

temperature and cause the water vapour to fall out and

either appear as condensation on the surface or

disappear inside the wall, wood, paper, leather whatever.

Or if its outside we see it as dew, frost, rain, hail or snow.

Perhaps you should think of molecules of water vapour

as being so small that they can freely move between the

surface molecules of most things used in building. Think

of a box full of footballs, the footballs being the air

molecules, then think of the molecules of water vapour

as being tiny ball bearings that can fill the large spaces

between those footballs, dependent on their

temperature.

Therefore, water vapour enters the bubbles of air that

are entrained into the mortar that is used to build our

homes and makes it way through the walls, etc towards

the cold on the other side to escape into the cold air.

Our traditional homes built to shed water allow this

process.

It is very difficult to make a wall or ceiling water vapour

proof.

There is a move to making modular rooms in factories

where the structure can be more carefully manufactured

and be closer to being water vapour proof.

In the normal world, all we can do is cover walls and

ceilings with almost water vapour proof plastic sheets

and keep most of the vapour inside our rooms or expel it

to the outside where it is usually colder and drier.

Some damp in a wall does no harm, and as mentioned

above, the water vapour will move outwards towards the

cold in winter and inwards towards the cold in summer if

there is air conditioning.

Keep in mind that we put water vapour into our homes.

Usually by washing, cooking, breathing and sweating.

The cooking and washing is usually in defined rooms

which can be carefully equipped and constructed to deal

with this problem.

Our living rooms are usually dry apart from breathing and

sweating, young people and animals sweat and breath

more as they tend to be more active, but they also tend

to be outside more.

Older people are less active, create less water vapour

and tend to be indoors more.

From the above you will see that the traditional solid wall,

drafty home, with an open fire was ideal for handling

water vapour.

Modern homes in our attempts to save fuel are sealed,

perhaps nearly air tight, retaining more water vapour.

Todays logic, seal your home to save on heating costs!

Pay a small fortune on buying, fitting and running

controlled mechanical ventilation.

You can use the traditional method, open a window for a

few minutes and let the warm wet air out and colder drier

air in.

At the point where the external insulation meets the solid

wall, you are furthest away from the room heat source

and at the coldest point in the home. This is where the

water vapour condenses.

Where you are trying to save money on heating your

home it makes sense to have the insulation as close to

your comfort zone as possible, keeping the comfort

zone as small as possible.

Spending good money on heating the mass of your brick

walls.

There is little point in adding the mass of your brick walls

to your heating bill, something you have to warm, before

your room feels warm.

Having the usual insulations on the room side of the wall

makes sense, if the room side water vapour barrier is not

perfect the escaping water vapour merely passes

through the wall.

However, some internal insulation, is flimsy, some is

easily damaged or marked, these need careful

consideration before use as visitors and children do not

always understand how easily damaged they are.

Covering the outside of a home with slates or tiles that

shed water, keeping the solid wall dry, helps with the

insulation. Water is 4000 times better at transferring heat

than dry air, a wet or damp wall is a liability.

There is a different type of open cell, self draining

insulation called Icynene, this is wind proof, it allows both

water vapour and water through its cells, when it gets

wet, the water simply drains down and disappears, water

vapour passes through without hindrance, yet it is as

good an insulation as most currently available, being

99% air.